Also the cable outer would protect your frame from the movements of the cable and protect the cable from any humidity in the tubes (i think?)
protect the cable from any humidity in the tubes
If there's humidity in your tubes which is going to trouble stainless steel brake cable, you should be worrying about the tubes, not the cable.
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Also the cable outer would protect your frame from the movements of the cable and protect the cable from any humidity in the tubes (i think?)